1. Field of the Invention
The present invention relates generally to vibration generation for mechanically stressing a product, an example of which is environmental stress screening. More particularly, the present invention is directed to a vibration hammer for generating random vibration.
2. Related Art
Environmental stress screening is a process by which a product is stressed by environmental conditions in an attempt to transform latent defects into manifest defects. This process allows a manufacturer to uncover flaws in product quality before the product is shipped to a customer. One environmental stress screening test which has proven effective is vibration.
In the infancy of vibration testing, manufacturers relied on swept sinusoidal vibration excitation. In swept sinusoidal testing, a single frequency sinusoidal excitation is swept through a frequency range of interest. A major disadvantage of these swept sinusoidal excitation systems is that the excitation frequency coincides with the natural resonance frequency of the unit-under-test for only a short period of time. Thus, there is insufficient vibration time at critical frequencies for latent defects to become manifest defects.
Recently, electrodynamic shakers have been used to generate random vibration excitation. Electrodynamic shakers consist of electrically-driven armatures, power amplifiers, and associated control equipment. Although electrodynamic shakers can generate random vibration, the cost is prohibitively expensive. This high cost has driven the environmental stress screening community to adopt pneumatic hammers as the preferred vibration source.
The conventional pneumatic hammer includes a housing and a piston. The piston is free floating within a cylinder of the housing and is pneumatically driven to reciprocate in the cylinder. The reciprocating motion along with collisions between the piston and an end of the cylinder generate the desired vibrations. A programmer (made from a polymer or other resilient material) may be attached to the head of the piston or the end of the cylinder to cushion the collisions between housing and piston.
Pneumatic hammers generally operate at a fixed frequency. They rely on the harmonic effects of the piston impacts to distribute excitation energy over a broad range of frequencies. A major disadvantage of pneumatic hammers is the high amplitude acceleration peaks at the fixed frequency of the hammer. If one or more of the natural frequencies of the unit-under-test is coincident with these acceleration peaks, then the unit may be excessively vibrated. Excessive vibration can overstress and damage the unit-under-test.
Several attempts have been made to randomize the operating frequency of pneumatic hammers. One method to randomize pneumatic hammers is to randomly regulate the pressurized gas used to drive the piston. This causes the hammer to excite the vibration table at varying intervals and with varying force which produces pseudo-random excitation. However, a true random vibration is not achieved.
Another method varies the stroke of the piston. Varying the stroke of the piston varies the interval and the force of the piston impacting the housing. However, this method produces only a limited pseudo-random vibration.
Thus, conventional methods have increased the randomness of pneumatic hammers, but have failed to produce true random excitation. Furthermore, the apparatus required to produce these pseudo-random effects is complex and often expensive to implement and manufacture.